Yeah, are you after graduated or just a solid ND to lengthen exposure times?
0.3 or 0.6 as already suggested are probably all you need to start with of either graduated or normal.
Again - as suggested take a look at the Cokin P-Series.
If you are after ND grads, think about whether you will need hard or soft transitions. Hard are best if you have a horizon running the whole way across your shot, whereas soft are better if you have things 'in the way' of the horizon.
There are many situations where there is too much in the shot that makes an ND grad useless, as you'd end up with the tops of mountains being darkened compared to the bottom, for example.